Question

Remove all grouping symbols and combine like terms,
a - { a - [ a- (2a - b) + 3a ]}

Answer 1

$a -\{ a - [ a- (2a - b) + 3a ]\}=\\ a -\{ a - [ a- 2a + b + 3a ]\}=\\ a -\{ a - [ 2a+b ]\}=\\ a -\{ a - 2a - b \}=\\ a -\{ - a- b \}=\\ a+a+b=\\ 2a+b$

Answer 2

a - { a - [ a- (2a - b) + 3a ]}
= a - {a - (a - 2a + b + 3a)}
= a - {a - (2a + b)}
= a - (a - 2a -b )
= a - (-a - b)
= a + a + b
= 2a + b